The mechanosensory lateral line, a primitive vertebrate hair cell system characteristic of all fishes and some amphibians, has long been established as a useful model for understanding how related hair cell systems, including the mammalian auditory system, work. The superficial location of lateral line hair cells on the body of fish offers significant advantages in terms of accessibility and ease of experimental manipulation. The lateral line system is not unlike an uncoiled cochlea along the body of the fish that encodes spatial rather than spectral variations in the stimulus field. Although the proximal stimulus field for the lateral line system is fluid flow past the fish rather than the traveling wave along the basilar membrane, information about the stimulus field and its source in both systems is encoded by excitation patterns along spatially-distributed hair cells. Furthermore, first-order brainstem nuclei in these two systems possess similar organizations, suggesting that they process information in a similar fashion. The long term objective of this research is to increase our understanding of how spatial patterns of variation along hair cell arrays are used by the nervous system to gain information about stimulus sources. In particular, we will investigate the importance of lateral line excitations patterns in extracting information about the location of external sources of water disturbance. A combination of anatomical, neurophysiological, behavioral, and modeling techniques will determine what happens in the central nervous systems between the time the lateral line system is stimulated and the time there is a behavioral reaction to the sensory input. Stimulus inputs, flow patterns about a vibrating sphere, are specified with a variety of stimulus measurement and computer modeling techniques, which are also used to predict excitation patterns along the lateral line. Naturally-occurring and conditioned behaviors are measured to determined the ability of fish to locate dipole sources. The same dipole sources are used in neurophysiological experiments, which measure the responses of both peripheral nerve fibers and ascending projection neurons in the first- order brainstem nucleus. Anatomical techniques are additionally being used to characterize the peripheral and central substrates that give risk to the input/output characteristics of the system. Intra-specific comparisons among individuals with various sensory systems intact and blocked, as well as inter-specific comparisons among different fish with different natural behaviors and sensory specializations, will be used to determine the relative roles of the lateral line and auditory systems in enabling fish to localize sources of water disturbance.